Hematopoietic stem cells (HSCs) ensure the production of all mature blood cells during homeostatic and regenerative hematopoiesis. It is clearly in the best interests of the organism, therefore, to evolve mechanisms that will protect HSCs from untimely death due to potentially lethal DNA damage. Work from this laboratory has shown that in response to DNA damage, the transcription factor Slug is induced by (-irradiation through p53 and protects committed hematopoietic progenitors from apoptosis triggered by otherwise lethal doses of radiation. Slug exerts this protective effect by repressing Puma, a proapoptotic target of p53. Since Slug is also expressed by HSCs under steady-state conditions, we propose that the radioprotective mechanism we have described for committed hematopoietic progenitors may also operate in HSCs, not only after radiation exposure, but also to maintain HSC functional integrity during steady-state hematopoiesis. Two specific research aims have been devised to test these predictions. First, since Slug-/- mice have a decreased number of functional HSCs under homeostatic conditions, we will establish whether this defect impinges on HSC homing, transition from quiescence to rapid proliferation under conditions of stress, or capacity for self-renewal. Second, we will test the hypothesis that Slug protects normal HSCs from the lethal effects of radiation-induced DNA damage by repressing the cell death protein Puma. On completion of these aims, we will have a much improved understanding of the physiological role of Slug in normal HSCs. If this transcriptional repressor proves important in inhibiting apoptosis, we will consider ways to manipulate its expression and/or function in order to better protect HSCs from genotoxic agents or stress conditions imposed by other factors. One obvious possibility would be to develop strategies to alter the Slug-mediated survival pathway so that it would sustain HSCs during cancer chemotherapy, thus ameliorating the life-threatening consequences of treatment-induced pancytopenia. Lay Summary: Blood stem cells can produce each of different blood cell types in the body. Scientists at the Dana-Farber Cancer Institute have discovered a protein called Slug that may protect these stem cells from damage that could harm their ability to make fresh blood components. If Slug does have this role, it could be an important target molecule in new treatments for blood diseases.